In cellular telephone systems, the served area is divided into cells, each of which may be further divided into sectors. Each cell is served by a single base station, or cell site, and each base station is connected to a message switching center ("MSC") via appropriate hardware links. A mobile unit is connected to the MSC by establishing a radio frequency ("RF") link with a nearby cell site.
Currently, there are several different types of cellular access technologies for implementing a cellular telephone network, including, for example, code division multiple access, or "CDMA", and advanced mobile phone service, or "AMPS". In a CDMA network, a single radio frequency is used simultaneously by many mobile units and each mobile unit is assigned a "code" for deciphering its particular traffic on that frequency. In contrast, in an AMPS network, each mobile unit is assigned a different radio frequency on which to communicate.
Referring to FIGS. 1A-1D, it will be recognized that, as shown in FIG. 1A, in order for a mobile unit, such as the mobile unit 10, to communicate in a cellular telephone network, designated generally by a reference numeral 12, two links must be established, including an RF link 14 between the mobile unit 10 and a first cell site, such as a cell site A, and a hardware link 16 between the cell site and a mobile switching center ("MSC") 18. As shown in FIG. 1B, as the mobile unit 10 moves away from the cell site A, the RF link 14 will eventually become too weak to support communications between the cell site A and the mobile unit 10 and will eventually disconnect, resulting in the call in progress being dropped. Clearly, this is not acceptable. Accordingly, as illustrated in FIGS. 1C and 1D, to avoid this result, as the mobile unit 10 nears a second cell site, in this case, a cell site B, a new communications path between the mobile unit 10 and the MSC 18, comprising an RF link 20 and a hardware link 22 between the cell site B and the MSC 18, is established. At this point, the mobile unit 10 is directed to end communication with the cell site A and begin communication with the cell site B.
Although as shown in FIGS. 1A-1D, the cell sites are connected by a single MSC 18, it will be recognized that cell sites may be connected by more than one MSC connected to one another via appropriate links. Moreover, the cell sites A and B may be located in two separate cellular telephone networks, such as a CDMA and an AMPS network, respectively, or in two different CDMA networks.
The situation in which a mobile unit ends communication with one cell site and begins communication with a second cell site, is referred to as a "handoff". The specific example illustrated in FIGS. 1A-1D is referred to as a "hard handoff", because the link between the mobile unit 10 and the MSC 18 via the cell site A is broken before the link between the mobile unit and the MSC via the cell site B is established.
In many cases, an AMPS cellular network already exists in an area in which a CDMA network is to be installed, in which case the CDMA network will often be overlaid on top of the AMPS network. In such cases, it is conceivable that there will be situations in which it would be advantageous to effect the handoff of a call from a CDMA cell site to an AMPS cell site. For example, it is possible that there will be areas that are covered by the AMPS network that are not covered by the CDMA network, due to different propagation characteristics of the two technologies, slight differences in the location of the various cell sites, and holes, or nulls, in the CDMA network coverage due to topography and cell site planning. Such coverage holes in the CDMA network can degrade call quality and, at worst, result in dropped calls. In addition, many cellular service providers have existing in-building and/or underground AMPS network coverage. The RF propagation characteristics of an 800 MHz signal in an AMPS network may be able to penetrate such structures better than a 1900 MHz signal in a CDMA network; therefore, a call maintained by the CDMA network could lose quality or be dropped. Finally, because large urban areas are typically converted to CDMA before rural areas, the major highways between such areas will also be converted to CDMA, to provide seamless coverage for travelers between such areas. However, as a mobile unit exits the highway, it also exits CDMA coverage area, eventually resulting in a call-in-progress being dropped.
In the foregoing situations, it would be beneficial to handoff a call from a CDMA network to an AMPS network on which it is overlaid once the call has degraded to a point at which it appears that the call will eventually be dropped.
It will be appreciated that CDMA network service providers would like to maintain a call within the CDMA network rather than hand it off to an AMPS network, as handing off a call too soon results in lost revenue for the CDMA network service provider; however, waiting too long to do so will likely result in a decrease in call quality and an increase in dropped calls, both of which result in an increase in customer complaints.
Therefore, what is needed is a method of triggering handoff of a call from a CDMA cell site to a second cell site, such as an AMPS cell site, responsive to a determination that the RF link between the mobile unit and the CDMA cell site has degraded to such an extent that a disconnection of the link is imminent.